1. Field of the Invention
The present invention relates to an optical fiber amplifier, and more particularly to an optical fiber amplifier for amplifying optical signals of the wavelength band of 1530 nm to 1610 nm, and a method therefore.
2. Description of the related art
Optical fiber amplifiers are the devices for directly amplifying optical signals without converting them into electrical signals, so that they do not employ optoelectronic repeaters, simplified in structure and economical in cost. They consist of active optical fiber, pumping lasers, optical couplers for providing combined optical signals and pumped light (hereinafter referred to as xe2x80x9cpumpxe2x80x9d) to the active optical fibers, and optical isolators.
The active optical fiber is doped with Er (Erbium[68]), Pr (Praseodymium[59]), and/or Yb (Ytterbium[70]) of the rare-earth group in the core, providing stimulated emission. See U.S. Pat. No. 5,892,781 to Jing-Jong Pan et al. entitled High Output Fiber Amplifier/Lasers For Fiberoptic Networks. The pump provided by the pump laser stimulates the rare-earth element doped in the core into highly ionized state, which makes the emission to amplify an optical signal coming into the active optical fiber. The widely used WDM (Wavelength Division Multiplexing) optical transmission system chiefly employs the wavelength band of 1550 nm (about 1530 nm to 1560 nm) as the signal band, and thus the erbium doped fiber amplifier (EDFA) especially suitable for amplifying an optical signal of the 1550 nm wavelength band.
FIG. 1 is a graph showing the gain characteristics of the incoming optical signals of an EDFA according to wavelengths based on the ratio of population inversion of Er+3 ions, where the gain characteristics are graphed with the ratio of population inversion being 0%, 10%, 20%, . . . 100%. The state of the ratio of population inversion being 0% indicates that all erbium ions are in the ground state, where the gain characteristics represent the maximum power loss around 1530 nm wavelength due to light absorption, as shown in FIG. 1. As the ratio of population inversion increases up to 100%, where all erbium ions becomes excited, the gain approaches the maximum value around 1530 nm wavelength. Hence, the ratio of population inversion of erbium ions is usually maintained at a suitable level of 70 to 100% to amplify the optical signals of 1550 nm wavelength band of 1530 nm to 1560 nm.
As shown in FIG. 1, however, the gain characteristics are different along the wavelengths for each ratio of population inversion. Namely, 1530 nm wavelength has the highest gain, and 1560 nm wavelength the lowest gain. Such gain irregularity along the wavelengths may be flattened by employing various means such as an optical filter for properly dampening the optical signals of 1530 nm wavelength region.
Recently, a wide-band WDM optical transmission system has been developed to use 1580 nm wavelength band of 1575 nm to 1605 nm as the signal band including 1550 nm wavelength band. Hereinafter, the 1550 nm wavelength band is called xe2x80x9cC-bandxe2x80x9d, and the 1580 nm wavelength band xe2x80x9cL-bandxe2x80x9d. Referring again to FIG. 1, when the ration of population inversion of erbium ions in EDF is maintained around 30% to 40%, there is obtained a gain flattened bandwidth of about 30 nm having a small gain in the 1580 nm wavelength band. Since the gain per unit length of EDF is very smaller in 1580 nm wavelength band than in 1550 nm wavelength band, the length of EDF for amplifying the optical signals of 1580 nm wavelength band should be about 10 to 20 times the length in the case of 1550 nm wavelength band.
To this end, there has been developed a wide-band EDFA for amplifying the optical signals of both C-band (1550 nm wavelength band) and L-band (1580 nm wavelength band), which comprises an L-EDFA for amplifying the optical signals of the L-band arranged in parallel with a C-EDFA for amplifying those of the C-band. Referring to FIG. 2 for illustrating such parallel structured wide-band EDFA, the conventional wide-band EDFA 10 comprises an L-EDFA (Long-band EDFA) 14 for amplifying the optical signals of the L-band arranged in parallel with a C-EDFA (Conventional-band EDFA) 12 for amplifying the optical signals of the C-band. The optical signals coming into the input terminal 15 are separated by an input 1550/1580 nm WDM optical coupler 16 into the optical signals of the C-band and those of the L-band, which are respectively inputted to the C-EDFA 12 and L-EDFA 14, combined by an output 1550/1580 nm WDM optical coupler 18 and delivered to the output terminal 19.
Because such a wide-band EDFA should firstly secure the amplifying performance, there have been provided various means including a high power-pumping laser in order to improve the amplification of the L-EDFA 14 having an inherently low amplification factor. Additional techniques for improving amplification are discussed in U.S. Pat. No. 6,049,417 to Atul Srivastava et al. entitled Wide Band Optical Amplifier, and U.S. Pat. No. 6,049,417 to Atul Srivastava et al. entitled Noise Figure In Optical Amplifiers With A Split-Band Architecture.
It is an object of the present invention to provide a wide-band optical fiber amplifier for achieving high amplification factor in the 1580 nm wavelength band (L-band) by a simple construction, and method therefor.
It is another object of the present invention to provide a wide-band optical fiber amplifier for effectively amplifying the optical signals of 1580 nm wavelength band by using backward ASE (Amplified Spontaneous Emission) generated during amplifying the optical signals of 1550 nm wavelength band.
According to an aspect of the present invention, a method for amplifying the optical signals of 1550 nm wavelength band and 1580 nm wavelength band in a wide-band optical fiber amplifier, comprises the steps of separating the incoming optical signals into 1550 nm wavelength band and 1580 nm wavelength band, respectively amplifying the optical signals of the 1550 nm wavelength band, and those of the 1580 nm wavelength band, and supplying the backward ASE generated in amplifying the optical signals of the 1550 nm wavelength band as a supplementary pumping light to the amplification of the optical signals of the 1580 nm wavelength band.